I am playing Two Worlds 2 right now as a mage. The spell making isn't really all that advanced. You just keep adding cards and hope to find a combination for a 'pre-made' spell. You might be able to increase its strength slightly with more cards, but its still a pre-made spell with a few options.
Still, customizing spells is EVERYTHING to people that like to play mages and the more complicated the better, IMO. After all, mages are supposed to have studied long and complicated processes to be able to handle magic. Its why the average commoner isn't throwing fireballs around all day.
I would love to see a system that allows the mage to create spells in a very customizable way. If I want my fireball to burn a little hotter, maybe I can sacrifice a bit of my own health to fuel it. Maybe I can make spells more powerful if I use a longer incantation so they take longer to cast. Maybe I could use a ritual consuming exotic materials back at my laboratory to create a one use stone that does some pretty amazing things. Maybe if I create a spell that is much more powerful than I could normally cast, but could only use it where something in the environment is present, like making an existing campfire explode in a dramatic fashion rather than just conjuring fire out of thin air.
That would be the magic-system I could play with all day.
How advanced is 2 worlds 2 spell making compared with Oblivion? Your description makes it sound far simpler.
Oblivion / Morrowind were pretty decent because you could specify target, strength, area, and duration for each effect and have lots of effect in a spell.
Your system sounds even more advanced as you can increase strength by increase casting time or disadvantages.